1. Field of the Invention
The present invention relates to a wet-type image formation apparatus, such as a wet-type electrophotographic copying apparatus, and in particular to a wet-type image formation apparatus in which a latent electrostatic image formed on an electrophotographic photoconductive member or electrostatic recording member is developed with a liquid developer.
2. Discussion of Background
In the conventional wet-type image formation apparatus such as a wet-type electrophotographic copying apparatus, visible toner images are formed on a transfer sheet in such a manner that an electrophotographic photoconductor, usually drum-shaped, is uniformly charged to a predetermined polarity by a main charger and exposed to light images which are converted from original images by an optical scanning system. Thus, latent electrostatic images are formed on the surface of the photoconductor. The thus formed latent electrostatic images are developed to visible toner images with a liquid developer. The visible toner images formed on the photoconductor are transferred to a transfer sheet and fixed thereto using a heat-application roller.
The liquid developer used in the above-mentioned wet-type development process is prepared by dispersing toner particles in a carrier liquid. For the carrier liquid, petroleum aliphatic hydrocarbons such as nonane, decane, dodecane, isooctane, isododecane and ligroin are usually used in combination.
The surface of the above-mentioned heat-application roller is generally coated with a release agent such as a silicone oil to prevent the transfer sheet from adhering to the heat-application roller in the image-fixing operation.
Recently, many functions are performed by the copying apparatus. For instance, a duplex or double-sided copying operation and a synthetic copying operation can usually be carried out by the copying apparatus. The double-sided copying operation is performed in such a manner that first one side of a transfer sheet then the other is subjected to the copying operation. In the synthetic copying operation, after a copy has been formed on one side of the transfer sheet, the same side thereof is again subjected to the copying operation, so that images are overlaid on the preceding images. These two kinds of copying operations have the characteristics that one transfer sheet is repeatedly subjected to the copying operation in common.
These copying operations have the drawback that a small amount of the release agent which has been applied to the heat-application roller is deposited on the surface of the transfer sheet during the image fixing of the preceding copying operation. When the transfer sheet on which a slight amount of the release agent is deposited is subjected to the succeeding copying operation, the release agent inevitably comes in contact with the surface of the photoconductor and adheres thereto. The release agent which has adhered to the surface of the photoconductor is removed therefrom when the residual liquid developer deposited on the photoconductor is cleared therefrom in a cleaning unit after development. Thus, the release agent is collected and returned to a liquid developer reservoir together with the residual liquid developer. As the double-sided copying operation or synthetic copying operation proceeds for an extended period of time, the amount of the release agent which is mixed with the liquid developer is gradually increased and a bubbling problem is produced in the liquid developer. This causes the liquid developer to flow to the outside of the development unit and the squeezing properties of a reverse squeezing roller to deteriorate.